Such devices use a signal-producing unit, which produces high-frequency measuring signals, a coupling unit, and a surface waveguide, wherein the measuring signal is coupled by way of the coupling unit onto the surface waveguide and guided thereon in the direction of the fill substance, and a receiving-/evaluation-unit, which directly or indirectly determines, on the basis of the travel time of the measuring signal reflected on the surface, respectively interface, the fill level of the fill substance, or the location of the interface, in the container. Such devices are generally known under the label TDR fill level measuring devices. Used as measuring signals are either short, electromagnetic, high-frequency pulses, or bursts, (TDR method or pulse-radar method), or continuous, frequency-modulated microwaves (e.g. FMCW-radar method), guided along the surface waveguide. Known variants of waveguide are: Sommerfeld or Goubau or Lecher waveguides. TDR fill level measuring devices working on the basis of guided microwave pulses are, moreover, marketed by the assignee under the designation Levelflex.
Considered physically, a TDR measuring device makes use of the effect that a part of the guided pulse, respectively guided microwaves, is reflected at the boundary between two different media, e.g. air and oil, or air and water, due to the abrupt change (discontinuity) in the dielectric constants of the two media, and is guided over the surface waveguide back into a receiving device. The reflected fraction, i.e. the useful echo signal, is greater, the greater the difference between the dielectric constants of the two media. Using the travel time of the reflected fraction of the high-frequency pulse, respectively the FMCW-signal, i.e. the echo signals, the distance to the surface of the fill substance can be determined. With knowledge of the distance to the bottom of the empty container, the fill level of the fill substance in the container can be calculated. If an interface determination is to be performed, the measuring results enable correct determination of the location of the interface.
Measuring devices with guided, high-frequency signals (pulses, bursts or waves) distinguish themselves, compared to measuring devices that freely radiate the high-frequency pulses or waves (free-field microwave systems (FMR), respectively ‘real radar systems’), by a significantly higher echo amplitude. The reason for this is that the power flow can occur completely targeted along the surface waveguide. Additionally, the measuring devices with guided high-frequency signals have a higher measuring sensitivity and measuring accuracy in the near region as compared with freely radiating antennas.
The length of the surface waveguide is usually selected such that the maximum measuring range within the container is covered. A waveguide with desired dimensions is either provided by the manufacturer, or the customer modifies the waveguide on site to the desired length. Disadvantageous in this is that a customer must limit itself to a certain measuring range, such as was ordered, or such as was chosen during the initial installation. If it is desired subsequently to modify the device for a container of different dimensions, then the required changes can be rather costly. In the worst case, a new surface waveguide must be purchased.